Gasoline is generally prepared from a number of hydrocarbonaceous blend streams such as butanes, light straight run naphtha, isomerate, fluidized catalytically cracked gasoline, hydrocracked naphtha, coker gasoline, alkylate, reformate, ethers and alcohols. Of these, gasoline blend stocks produced in fluidized catalytic cracking (FCC) units, reformer units and alkylation units account for a major portion of the world's gasoline pool. FCC gasoline, and if present, coker gasoline and pyrolysis gasoline, generally contribute to a substantial portion of the concentration of organic sulfur in the gasoline pool.
Organic sulfur present in the gasoline pool may be in one of several molecular forms, including thiophenes, benzothiophenes, mercaptans, sulfides and disulfides. Typical thiophenes and benzothiophenes include their alkylated derivatives. Of particular interest are 2-methylthiophene, 3-methylthiophene, 2-ethylthiophene and dimethylbenzothiophene. Typical mercaptans occurring in the sulfur-containing gasoline streams include thiophenol and alkylthiols from ethanethiol to decanethiol with potentially smaller amounts of the higher alkylthiols. Mercaptans of particular interest include 1-ethanethiol, 2-propanethiol, 2-butanethiol, 2-methyl-2-propanethiol, pentanethiol, hexanethiol, heptanethiol, octanethiol, nonanethiol and thiophenol. Sulfides and disulfides in the gasoline pool may be the result of thioetherification, mercaptan oxidation or extraction treatments upstream.
Many gasoline blend streams also contain olefins which tend to have a higher octane value than their paraffinic counterpart. Blend streams originating from an FCC unit, for example, will be olefinic, with an olefin content of at least 5 or more percent, and typically in the range of 10 to 30 percent.
A number of methods have been proposed for removing sulfur from gasoline. In general, hydrotreating is very popular because of the cost and ease of processing using a catalytic hydrotreating method. However, sulfur removed by hydrotreating is generally accompanied by a substantial reduction in octane number because the olefins are saturated to paraffins.
In order to meet new stricter gasoline sulfur specifications while minimizing octane loss, most refiners will end up fractionating the FCC gasoline into at least two streams. The overhead stream will be rich in olefins and contain a low fraction of the total sulfur from the feed. Most of the sulfur present in the overhead stream will be in the form of mercaptans, disulfides and dissolved hydrogen sulfide.
U.S. application Ser. No. 09/901,215 filed by the assignee of the present invention, and incorporated herein by reference, discloses a process for desulfurizing a gasoline stream containing sulfur compounds and olefins by fractionating it into three fractions. The low-boiling fraction is subjected to mercaptan oxidation to remove at least a portion of the mercaptan sulfur compounds. The mid-boiling fraction containing thiophene and olefins is subjected to solvent extraction to produce a product raffinate stream containing olefins and having a reduced sulfur content and a rich solvent stream enriched in thiophene. The rich solvent stream enriched in thiophene is separated to produce a hydrocarbonaceous stream rich in thiophene. The resulting hydrocarbonaceous stream rich in thiophene and the higher boiling fraction containing sulfur compounds are reacted in a hydrodesulfurization reaction zone to produce a hydrocarbon stream with a reduced sulfur content.
The paper titled, “Removal of Sulfur from Light FCC Gasoline Streams,” presented at the NPRA 2000 Annual Meeting on Mar. 26–28, 2000 in San Antonio, Tex., discloses that sulfur compounds in the initial boiling range of light FCC gasoline are primarily mercaptans which are extractable in a mercaptan oxidation process.
An extraction mercaptan oxidation process is described in D. L. Holbrook Handbook of Petroleum Refining Processes, 11.30–11.31 (Robert A. Meyers, 2d ed., 1997). In this process, a liquid hydrocarbon stream is pretreated to remove or dissolve hydrogen sulfide in caustic. The pretreated hydrocarbon stream is subjected to counter-current flow with caustic in an extractor vessel. Mercaptans in the hydrocarbon stream react with caustic to yield mercaptides which are soluble in the caustic. A product hydrocarbon stream lean in mercaptans passes overhead and a mercaptide-rich caustic passes from the bottoms. The mercaptide-rich caustic is mixed with air and catalyst to oxidize the mercaptides to disulfides. The caustic is separated from the disulfides and is returned to the extraction.
U.S. Pat. No. 5,244,643 B1 discloses a non-extractive mercaptan oxidation process whereby a hydrocarbonaceous gas stream including mercaptan sulfur, air and aqueous alkaline solution including a mercaptan oxidation catalyst are mixed in a mixing vessel in which mercaptans are converted to disulfides. The effluent withdrawn from the top of the mixing vessel is settled in a vessel to yield separate streams of air, liquid hydrocarbon product containing disulfides and an aqueous alkaline solution including catalyst.
U.S. Pat. No. 4,775,462 B1, which is incorporated by reference, teaches an alternative approach to removing mercaptan compounds from a hydrocarbon stream by thioetherification. In a thioetherification process the mercaptan compounds are reacted with olefinic hydrocarbons to form organic sulfides. Organic sulfides boil at higher temperatures than mercaptans and can be recovered in a high boiling stream of a fractionation column. Organic sulfides can also be completely converted to hydrogen sulfide and paraffins in a hydrotreating processes.
A paper titled, “Novel Process for FCC Gasoline Desulfurization and Benzene Reduction to Meet Clean Fuels Requirements,” presented at the NPRA 2000 Annual Meeting on Mar. 26–28, 2000 in San Antonio, Tex., discloses that sulfur and aromatic species in FCC naphtha may be segregated using solvent extraction.
U.S. Pat. No. 5,582,714 B1 discloses a process for the removal of sulfur from FCC gasoline by employing a solvent. Preferred solvents are glycols and glycol ethers. U.S. Pat. No. 2,634,230 B1 discloses a process for desulfurization of high sulfur olefinic naphtha wherein 2, 4-dimethylsulfolane is employed to extract sulfur from a highly olefinic naphtha, such that the solvent does not affect separation between olefins and paraffins. Instead, the process produces a sulfur-lean raffinate phase and a sulfur-rich extract. It is also known to use solids as adsorbents or reactants to remove sulfur from hydrocarbon streams.
U.S. Pat. No. 2,792,332 B1 discloses a process for the removal of aromatics and sulfur compounds from a feedstream comprising heavy naphtha, aromatics and sulfur compounds wherein the feedstream comprising heavy naphtha is contacted in a first extraction column with a solvent combination comprising isopropyl alcohol and polyethylene glycol to obtain a concentrated aromatic fraction and a paraffinic-naphthenic raffinate. The raffinate is first distilled to remove the alcohol, the resulting alcohol depleted raffinate is water washed to remove traces of the polyethylene glycol and then dried before it is recycled for reprocessing with the feedstream. The extract phase is similarly processed to first remove the alcohol by distillation and the alcohol-free extract is steam distilled to recover an aromatic product and to provide an aromatic-free polyethylene glycol/water stream. The polyethylene glycol/water stream is then passed to a solvent recovery tower to distill off the remaining water. In a second extraction column, the concentrated aromatic fraction is contacted with pure polyethylene glycol to recover an aromatic extract, which is steam distilled to provide a purified aromatic product comprising aromatic sulfur-type compounds.
U.S. Pat. No. 4,781,820 B1 and U.S. Pat. No. 4,498,980 B1 disclose processes for the separation of aromatic and non-aromatic hydrocarbons from a mixed hydrocarbon feed wherein the feedstream is contacted with a solvent comprising a polyalkylene glycol and a co-solvent comprising a glycol ether.
U.S. Pat. No. 5,928,497 B1, U.S. Pat. No. 4,337,156 B1 and CA 2,087,926 B1 teach processes for adsorption of sulfur-containing compounds. U.S. Pat. No. 6,156,084 B1 discloses reacting organic sulfur compounds out of a stream by contacting it with a solid nickel reactant.
According to U.S. Pat. No. 3,957,625 B1, sulfur impurities tend to concentrate in the heavy fraction of gasoline. A method for removing sulfur includes hydrodesulfurization of the heavy fraction of the catalytically cracked gasoline so as to retain the octane contribution of the olefins which are found mainly in the lighter fraction.
Most of the sulfur present in the bottoms stream is in the form of thiophene, benzothiophene and their substituted derivative compounds which cannot be removed from naphtha except by hydrodesulfurization. U.S. Pat. No. 6,228,254 B1 discloses extracting remaining sulfur compounds after mild hydrotreating to reduce the sulfur concentration of the gasoline to a very low level without significantly reducing the octane number of the gasoline.
A reforming process described in N. Dachos, A. Kelly, D. Felch and E. Reis, Handbook of Petroleum Refining Processes, 4.3–4.26 (Robert A. Meyers, 2d ed., 1997) involves contacting hydrocarbon feed such as naphtha with platinum-containing catalysts at elevated temperatures and in the presence of hydrogen at pressures ranging from 345 to 3450 kPa (50 to 500 psig) to produce a high-octane liquid product that is rich in aromatic compounds. Chemical hydrogen, light gas and liquefied petroleum gas (LPG) are also produced as reaction by-products. Catalyst is subjected to continuous regeneration to burn off coke and maintain its activity. Because reforming processes are typically preceded by a hydrotreater unit, sulfur is not substantially present in the reformate. Reformer units typically preceded by a hydrotreater reactor are usually a component of a refinery complex. The reformer unit typically processes feeds such as straight run heavy naphtha from a central distillation unit and heavy hydrocracked naphtha from a hydrocracking unit.
Dividing wall columns include a partition in the distillation column near the exit for the middle stream to control the composition of the middle stream more precisely than can a side-draw column. The tighter specification on the boiling range of the middle stream allows for a reduction in the flow rate of the middle stream and improved operational control. Moreover, dividing wall columns often provide reduced capital cost, plot space requirements, and energy cost compared to a sequence of two distillation columns. U.S. Pat. No. 2,471,134 B1 and U.S. Pat. No. 4,230,533 B1 disclose dividing wall columns and are incorporated herein by reference.
The recovery of thiophene in the overhead stream is a key operational parameter of the naphtha fractionation column. Thiophene is the lowest boiling sulfur compound that will not be converted in a caustic mercaptan oxidation process. Ideally, the fractionation column would be operated such that the maximum amount of olefin-rich material is recovered overhead, provided that the sulfur content remaining in the overhead stream after any treatment is low enough that the refiner can meet the overall sulfur specification for the gasoline pool.
There are several problems intrinsic to operating fractionation with high recovery of olefins overhead. The amount of thiophene present in the column feed is usually unknown and may vary depending on the operating conditions in the FCC process. Thiophene exhibits strongly non-ideal vapor-liquid equilibrium mixtures with many of the other components present in naphtha from an FCC unit. Additionally, Desty, D. H. and Fidler, F. A., “Azeotrope Formation between Sulfur Compounds and Hydrocarbons,” Industrial and Engineering Chemistry, 43(4), 905–910 (1951) reports that azeotropes are known to form between thiophene and many hydrocarbons with 6 to 8 carbons present in gasoline. Moreover, thiophene is difficult to detect in FCC naphtha. When the standard analysis of composition by gas chromatography is made, thiophene is aliased with sec-butyl mercaptan. Sec-butyl mercaptan is treatable using a caustic mercaptan oxidation process, but thiophene is not.
Because of these problems, a conservative design approach would be to operate the fractionation column at very low recovery of olefins overhead, sending a substantial part of the C6 olefins to the bottoms stream. However, this results in a reduction of octane number due to the saturation of C6 and C7 olefins. Moreover, the increased consumption of hydrogen caused by olefin saturation in the hydrotreating process would result in increased cost.
Accordingly, it is an object of the present invention to operate a three-way splitter fractionation column with post-treatment unit operations such that the total sulfur in all three streams leaving the three-way splitter does not exceed a sulfur contribution allocated to FCC gasoline. It is a further object of the present invention to provide a simple control scheme for governing the flow rates of the overhead, middle and bottoms streams from the three-way gasoline fractionation column so that the aggregate streams after post-fractionation treatment do not exceed the limits on sulfur contribution from the FCC process.